Manipulation of droplets and plugs using surface acoustic waves

Microfluidics, the study of fluid flow at the sub-millimetre scale, is an enabling technology that offers scaling down of common laboratory procedures into what is called lab-on-a-chip (LOC) devices. Such devices exhibit significantly reduced fluid consumption, faster reaction times and higher sensitivity. Droplet microfluidics, where nanoliter to femtoliter volume droplets are dispersed in an immiscible carrier medium, offers physical and chemical isolation of droplets so that they could be used as micro-reactors to study reactions. However, it's quite challenging to manipulate droplets in closed microfluidic channels. Researchers have designed passive as well as active manipulation techniques to accomplish tasks such as merging, sorting or mixing of droplets. Elegant and efficient as they may be, such systems fail to demonstrate cross-compatibility due to fabrication differences and imposed flow conditions. This thesis presents three novel droplet microfluidic devices capable of performing highest demand droplet manipulation techniques, starting with sensing and sorting to merging and splitting, in an integrable manner offering exceptional on-demand control using surface acoustic waves (SAWs). SAWs are nm-scale amplitude, MHz frequency waves that are generated by electrodes deposited on piezoelectric substrates. Microfluidic devices utilising SAWs are easy to fabricate and operate; they are portable, energy efficient and safe to biological samples. Droplet trapping and coalescence, steering at Y-junctions and splitting at T-junctions using SAWs were realised and thoroughly characterised in the presented thesis. They were designed in such a way that they could be mixed and matched in order to perform a specific study. Moreover, it was shown that a combinatorial library could be formed using an automated LOC device presented here that can sense, split, merge and mix droplets. These systems could easily be coupled with existing LOC devices or modified to carry out required workflows in an efficient manner.